Sealants are used to provide liquid and gaseous barriers in various applications. Such applications include bonding of dissimilar materials, sealing of expansion joints, assembling curtain walls and side walls, weatherproofing, constructing roofing systems, and sealing the perimeters around doors, windows and other building components (i.e., perimeter sealing).
Sealant compositions can be of the one-part or two-part variety. Moisture curable, one-part sealant compositions generally contain end-capped polyurethane prepolymers and, usually, a curing catalyst that promotes a cross-linking reaction between the prepolymers when the sealant composition is exposed to atmospheric moisture. Upon application under normal conditions of temperature and moisture, one-part sealant compositions react to form tough, pliable elastomeric seals.
Sealant and coating compositions desirably have a combination of properties which render them particularly suitable for their intended applications. Such compositions should be able to be packaged in sealed containers or cartridges and stored for relatively long periods of time without objectionably “setting up” or hardening (as a result of cross-linking). When applied as a caulking sealant or coating composition, they should form a relatively tack-free surface soon after being applied and exposed to atmospheric moisture, and should cure without the formation of bubbles within an acceptable time period. Such compositions should adhere tenaciously in the cured state to a wide variety of surfaces, such as to glass, aluminum, concrete, marble and steel surfaces. The sealant or coating in the cured state should have sufficient elasticity and flexibility to withstand expansions and contractions of panels, etc. with which it is associated during temperature variations that result from climatic changes, and to withstand wind forces that cause panels with which it is associated to flex or twist.
Polyurethane sealant and coating compositions typically are based on isocyanate-terminated prepolymers. In some cases, such prepolymers are fully or partially end-capped with silane groups. In general, sealants made with prepolymers that have been 100% end-capped with silane groups are not sufficiently flexible to sustain large joint movement. Accordingly, prepolymers that are end capped with a combination of silane groups and other groups, particularly aliphatic alcohol groups, have been developed. Although sealants made with prepolymers that have been end capped with a combination of silane and aliphatic alcohol groups have enhanced flexibility, these sealants are more likely to chalk, crack and yellow when exposed to extreme weathering conditions of high heat, high moisture, and prolonged exposure to UV radiation. Such sealants can also lose strength when exposed to these weathering conditions. Many of these undesirable changes can be attributed to reversion or degradation of the polymeric chains that are present in the cured sealant.
The polymeric chains of polurethane prepolyers that have been partially end-capped with aliphatic alcohol end cappers can degrade during storage, particularly when exposed to high heat. As a result of such degradation sealant compositions made with these prepolymers lose their mechanical strength and are, thus, less desirable.
Accordingly, it is desirable to have new sealant compositions that provide elastomeric seals with improved weathering stability. New prepolymer compositions that have enhanced heat and storage stability are also desirable.